I'll lay out the details then.
In doesn't matter what age you are. I guess 15 is an appropriate age. Middleman's carry out and look after trades of members to ensure that a trade goes safe and without scam. You basically need to speak English fluently and very well. You must be pretty active. You can only 'apply' to become a middleman when applications for them come out... Usually you'd see a thread for applications in the Middleman Lounge section... There aren't any up right now. You're going to have to wait for one. Also, don't expect to get chosen out of the blue like that, you should actually 'contribute' towards the marketplace section... A good example would be scammer's grave. A middleman's job is basically to carry out the entire scammer report. But members can also ditch in and help as well. You could ask members for maybe small little things they may need in a scammers report, for example, proof like real chat windows and not text written, give more information on the report, or point out some small things he may be generally missing... There are also basic requirements like the 800+ posts requirement, four or more months registered on this site, etc...